With the highly exothermic nature of some reactions, including the hydrogenation of certain fluorocarbons, there is a need to remove heat while a reaction progresses. Traditional shell-and-tube reactors often do not have enough heat transfer ability to keep the reaction systems at reasonable operating conditions. On the other hand, a purely trickle-bed reactor setup does not always provide enough ability to control both the temperature and/or reaction progression.
Generally, a shell-and-tube catalytic reactor is a type of reactor which is used to efficiently add or remove reaction heat. In some uses of these reactors, a catalyst is filled in a plurality of reaction tubes; a reaction fluid (gas and/or liquid) into the reaction tubes to cause a chemical reaction for obtaining a desired product; and a heat transfer medium is circulated through the reactor shell such that the chemical reaction can occur under controlled thermal conditions.
Shell and tube reactors typically include a number of reaction tubes held in place in the shell by one or more tubesheets; shell nozzles are used for introducing and withdrawing the heat transfer medium; tube nozzles are used for introduction of reactants into the reaction tubes and for withdrawing product therefrom; and appropriate dividers and/or baffles are used to separate the respective reactor parts for their specific functions. The reactor parts are typically made from materials that do not react with the materials being processed in the reactor.
The term trickle bed reactor is used to describe a reactor in which a liquid phase and a gas phase flow concurrently downward through a fixed bed of catalyst particles while reaction takes place. At sufficiently low liquid and gas flow rates the liquid trickles over the packing in essentially a laminar film or in rivulets, and the gas flows continuously through the voids in the bed.
A useful general review of trickle bed reactors and other multiphase reactors can be found under the heading “Reactor Technology” in “Kirk-Othmer Encyclopedia of Chemical Technology”, Third Edition, Volume 19, at pages 880 to 914. This reference states the following at page 892, “Trickle-bed reactors have complicated and as yet poorly defined fluid dynamic characteristics. Contacting between the catalyst and the dispersed liquid film and the film's resistance to gas transport into the catalyst, particularly with vapor generation within the catalyst, is not a simple function of liquid and gas velocities.”
In operation, a typical trickle bed reactor has a fixed catalyst bed positioned vertically. A reaction mixture comprising a liquid, a gas or both, flows downwardly through the bed. The exothermic heat of reaction is absorbed by vaporizing a combination of reactant, product, by-product, and optionally solvent. For a reaction to occur, the reactants must diffuse into the liquid phase, then diffuse to the catalyst particles, and then react. The reaction products, if soluble in the liquid phase, are then removed from the reactor.
The total reaction mechanism in such a system thus includes the steps of diffusion and reaction. The reactants must diffuse into the liquid phase and then diffuse to the catalyst particles, and the reaction rate is thus affected by the rate of diffusion to the catalyst particles. Assuming a situation where the catalytic reaction occurs at the surface of the catalyst particles, the reaction rate is affected further by the reaction rate constant, the concentration of the reactants at the particle surface, and the surface area of the catalyst particles. The resulting reaction products must then diffuse away from the catalyst particles and back to the mainstream liquid flow. Accordingly, the final reaction rate, as controlled by the slowest of the aforementioned steps, and is affected most by either the rate at which catalysis proceeds or the rate at which diffusion of the reactants and the products proceeds. The primary resistance to diffusion occurs at boundary layer areas, and thus it would be advantageous to increase both the gas and the liquid flow rates to decrease such boundary layers.